Metal nanogrids, nanowires, and nanofibers for transparent electrodes

Metal nanogrids, nanowires, and nanofibers for transparent electrodes Liangbing Hu, Hui Wu, and Yi Cui Metals possess the highest conductivity among all room-temperature materials; however, ultrathin metal films demonstrate decent optical transparency but poor sheet conductance due to electron scattering from the surface and grain boundaries. This article discusses engineered metal nanostructures in the form of nanogrids, nanowires, or continuous nanofibers as efficient transparent and conductive electrodes. Metal nanogrids are discussed, as they represent an excellent platform for understanding the fundamental science. Progress toward low-cost, nano-ink-based printed silver nanowire electrodes, including silver nanowire synthesis, film fabrication, wire-wire junction resistance, optoelectronic properties, and stability, are also discussed. Another important factor for low-cost application is to use earthabundant materials. Copper-based nanowires and nanofibers are discussed in this context. Examples of device integrations of these materials are also given. Such metal nanostructurebased transparent electrodes are particularly attractive for solar cell applications.

Introduction Transparent electrodes are ubiquitously used in optoelectronic devices where light must pass through a section, and current or voltage needs to be applied. These devices include thin-film solar cells (organic, inorganic, hybrid), displays (liquid-crystal display, plasma display, organic light-emitting diode [OLED]), touch screens (resistive, capacitive), electronic books, smart windows, fl exible lighting, and passive devices (such as transparent heating, transparent speakers).1 The properties of transparent electrodes are extremely important for device performance, and the specifications vary based on types of devices. For example, while source roughness and work function are insignificant for resistive touch screens, they are crucial for OLEDs and organic solar cells.1 Moreover, the optical haze of particular transparent electrodes is beneficial for high-efficiency solar cells but problematic for certain types of displays. The mechanical properties of the electrode are crucial for flexible devices but not important for devices on rigid substrates. Across all devices, however, the optical transparency (T ) and electrical sheet resistance (Rs) are the two most important parameters. Unfortunately, these two parameters are in constant competition with one another. Since Rs values are often reported at different transparencies, it is hard to compare two transparent electrodes directly, and hence a figure of merit (σdc/σop, where σdc is the dc conductivity, and σop is the optical conductivity)

is widely used to evaluate the performance of transparent electrodes.2,3 Depending on the application, Rs can range from 10 Ω/sq to 106 Ω/sq, but typically the transparency needs to be ≥90%. 106 Ω/sq is sufficient for antistatic applications, 400–1000 Ω/sq is sufficient for many touch screen applications, and ≤10 Ω/sq is needed for OLEDs and solar cells.1 Recently, Rowell and McGehee conducted a theoretical study on Rs for thin-film solar cell modules. Their conclusion was that a competitive transparent electrode must have an optical transparency of at least 90% at an Rs of less than 10 Ω/sq for monolithically integrated modules.3 Koishiyev and Sites conducted a similar study on the impact of Rs in 2D modeling of thin-film solar cells.4 Metal busbar fingers conduct current globally, and transparent electrodes conduct current locally. Assuming the current is distributed uniformly across the transparent electrode, the power loss associated with the sheet resistance of the transparent electrode is P = RsL2J 2/3, where P is the power loss, J is the current density, and L is sub-cell length.4 For high-efficiency solar cells, Rs needs to be as low as possible to reduce power loss from transparent electrodes. Over the past eight years, various types of transparent electrodes based on nanoscale materials have emerged. Percolative networks with randomly distributed carbon nanotubes (CNTs) have been extensively studied, and startups such as Unidym

Liangbing Hu, Department of Materials Science and Engineering at the University of Maryland at College Park, MD 20742, USA; [email protected] Hui Wu, Department of Materials Science and Engineering, Stanford University, CA 94305, USA; [email protected] Yi Cui, Department of Materials Science and Engineering, Stanford University, CA 94305, USA; [email protected] DOI: 10.1557/mrs.2011.234

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© 2011 Materials Research Society

METAL NANOGRIDS, NANOWIRES, AND NANOFIBERS FOR TRANSPARENT ELECTRODES

and Eikos have focused on commercializing the technology.2,5 However, due to the large tube-tube contact resistance, CNTbased transparent electrodes typically give the best performance of Rs of 150 Ω/sq and optical transparency of 80%.6 In the past four years, graphene-based transparent electrodes have been investigated, and an excellent performance of 30 Ω/sq Rs value and 90% optical transparency have been demonstrated for chemical vapor deposition-grown continuous graphene.7 For low-cost applications, solution-processed films with graphene flakes are more attractive, and the best performance until now is in the range of 200–300 Ω/sq and 80%.8 These carbon nanostructure–based transparent electrodes show excellent mechanical flexibility and are promising for touch screens and flexible devices. Unfortunately, their T/Rs performance does not meet the requirements for many other applications, such as solar cells. To achieve the highest performance in optical transparency and sheet resistance, metal-based nanostructures need to be used to their highest electrical conductivity at room temperature. In this review, we summarize recent research on nanostructured metallic transparent conductors, which includes structure design, fabrication, properties, device integration, future challenges, and the potential for flexible electronics and next-generation solar cells.

Metal thin films Due to their high free-electron density, metals have the highest conductivity among materials at room temperature. As another consequence, bulk metals are highly reflective in the visible range and cannot function as a transparent electrode. Like many other materials, metals become more transparent as the thickness decreases down to the tens of nanometers range. Such ultrathin metals have been explored as transparent electrodes and can in fact be purchased. Pipe and Shtein9 reported a summary of transparent electrodes based on nonpatterned metal films, where the Rs value is approximately 10 Ω/sq for a nonpatterned metal film with a thickness of around 10 nm. However, as film thickness decreased from approximately 20 nm, Rs sharply increased due to electron scattering from the surface because of substrate and grain boundaries. This mechanism has been previously investigated and is explained by the Fuchs-Sondheimer and Mayadas-Shatzkes models.9 As the thickness of the metal becomes less than 10 nm, the metal thin film becomes more transparent; however, the sheet resistance increases dramatically with decreasing thickness. This thicknessdependent transport makes thin metal films less attractive for transparent electrode applications. In a separate study, Pruneri et al. also investigated the potential of ultrathin metal films for transparent electrodes10 and found that a sputtered metal thin film with a thickness of less than 10 nm formed a uniform and continuous film over a 10 cm substrate. In addition, they found Ni films with a thickness of 2–10 nm have an average visible transmittance between 40–80% and sheet resistance between 30–1200 Ω/sq. These metal thin films are highly transparent in the UV and IR wavelength range and hence are attractive for many applications such

as IR detectors and IR solar cells. Transparent window films based on metals are commercially available, and both silver and gold films on plastic poly(ethylene terephthalate) substrates are also available. Compared with Ni film, these silver and gold films have a much better performance in terms of T/Rs due to their high electrical conductivity. Typical Rs and T values for transparent electrodes based on silver or gold is 4.5–9 Ω/sq and 75%, respectively.1

Metal nanogrids In an effort to achieve the best metal-based transparent electrode, nanostructures can be employed. One model structure is a metal nanogrid with periodic metal lines. Such nanostructures provide opportunities to manipulate photons and electrons to achieve unique properties that are not possible for a flat indium tin oxide (ITO) electrode. Because the thickness of the metal lines is much larger than that of metal films, electron scattering due to the substrate roughness and the strain boundaries decreases, and therefore the conductivity is close to that of the bulk counterpart. Light scattering and coupling in such nanogrids also provides additional benefits for device applications, particularly for solar cells where light scattering enhances the absorption of the active layer. As noted by Guo et al., two design considerations must be made for nanogrid-based transparent electrodes: (1) the line width of the metal mesh is subwavelength to provide sufficient transparency; (2) the period of the mesh is submicrometer to ensure the uniformity of the current across the active layer.11 Excellent performance has been achieved based on such metal nanogrids (Figure 1a and 1b). Catrysse and Fan performed a calculation with a finitedifference frequency-domain method for nanopatterned metallic films as a transparent conductor in optoelectronic devices and found an Rs value of 0.8 Ω/sq and a T value of 90% to be achievable for nanopatterned silver films.12 Moreover, Cui and Peumans calculated the optical electrical field and power flow for silver nanogrids (Figure 1a and 1b).13 Finite-element modeling was used to calculate the transmittance of metal nanogrids as a function of the geometric parameters. In Figure 1c and 1d, the line width is 40 nm, the period is 400 nm and the height is 100 nm, and the theoretical Rs is 1.6 Ω/sq. Figure 1e shows the transmittance for transverse electric (TE) and transverse magnetic (TM) modes from the modeling. Various methods for fabricating such metal nanogrids have been reported.11,14,15 Guo et al. pioneered nanoimprint lithography, and using this method, they demonstrated excellent results on various metals and substrates and also showed applications in solar cells and LEDs. Recently, they have developed largearea continuous imprinting of nanogrids using their apparatus that is capable of roll-to-roll printing on flexible web and roll-to-plate printing on rigid substrates.16 In addition, they developed a different nanopatterning method, localized dynamics wrinkling, which creates continuous metal nanogratings by simply sliding a flat edge of a hard material on a thin metal coated polymer layer.15 MRS BULLETIN • VOLUME 36 • OCTOBER 2011 • www.mrs.org/bulletin

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shows an Ag NW network. Charge transport happens along the Ag NWs. Due to the PVP coating on the surface, the junction resistance is typically high, ∼1 GΩ, and limits the overall conductance of the network. Various methods to decrease the junction resistance have been developed, such as electrochemical annealing, heat treatment, and rapid thermal annealing.18 Resistance of a single junction between two nanowires can be monitored in a device with metal contacts fabricated by e-beam lithography (Figure 2d). For example, Rs decreases dramatically (from >1000 Ω/sq to 100 Ω/dq) upon heating at 200ºC for 20 minutes, due to the fusion of the Ag-Ag junction through the insulating PVP layer. Further heating leads to increases in the resistance, which is due to the coalescence of the Ag NWs. Cui et al. performed a comprehensive study on scalable coatings and the properties of Ag NWs, including the flexibility Figure 1. (a) Scanning electron microscopy image of an as-fabricated metal electrode. (b) Average transmittance versus sheet resistance of metal grids with a line width of 120 nm.17 of different bending radius, adhesion, and staAbsolute value of the modeled optical electric field and power flow for light (600 nm bility toward various chemicals/temperature.18 wavelength) incident on a freestanding Ag grating (400 nm period, 40 nm line width, Solution-processed Ag NW electrodes show 100 nm high) for both (c) transverse magnetic (TM) and (d) transverse electric (TE) polarization. (e) Simulated transmittance versus wavelength for two modes. The AM1.5 excellent performance in terms of transparency solar irradiance data (black line) are plotted for comparison. and sheet resistance. Coleman et al. prepared films with an Rs value of 85% and a T of 13 Ω/sq Toward low cost: Solution-processed silver and showed the films to be electromechanically robust (no nanowire network change in resistance after being bent more than 1000 cycles).19 While metal nanogrid-based transparent electrodes demonstrate Figure 2e shows the specular transmittance of Ag NWs with excellent performance and are ideal for fundamental simudifferent Rs. The sheet resistance and transparency in the visible range of the Ag NW electrode are comparable to those lations, the fabrication process is often costly, and vacuum of the ITO electrode. Also the transmittance curve of the Ag deposition is needed, making them unrealistic for practical NW electrode is flat in the near infrared regions, while the applications. For low-cost applications, solution-based hightransmittance for the ITO electrode decreases for wavelengths speed roll-to-roll processing is preferred. Recently, networks of >1100 nm (Figure 2g). One figure of merit for comparing difrandomly distributed metal nanowires, especially silver nanoferent transparent electrodes is σdc/σop, which was first used by wires (Ag NWs), have been explored as an emerging candiGruner et al.2 Here, σdc and Rs are highly thickness-dependent. date since these networks maintain the advantages of patterned Coleman studied the thickness dependence of Rs, σdc, and σdc/σop nanogrids such as high transparency, low sheet resistance, and and found that a high figure of merit, 500, was achieved for mechanical flexibility.13 The keys to this technology include the growth of Ag NWs with small diameters in solution, stable thick Ag NW films.19 This is much higher than other transparent electrodes (nanotube ∼25 and graphene ∼0.5). nanoink formulation, scalable coating, decrease of junction resistance, and patterning. Ag NWs were synthesized by the Toward low cost: From silver to copper reduction of silver nitrate in the presence of poly(vinyl pyrrolSolution-processed silver nanowire thin films exhibit electrical idone) (PVP) in ethylene glycol.13 Figure 2a shows an Ag NW ink in methanol with a concentration of 2.7 mg/mL. The distriand optical properties comparable with ITO. However, silver butions of nanowire length and diameter are shown in Figure 2b. is also similar to ITO in price ($500/kg) and scarcity. Copper The percolation theory for one-dimensional sticks predicts that possesses a comparable conductivity with silver, is 1000 times the percolation threshold, Nc, dramatically decreases as the more abundant than indium or silver, and is 100 times less length of the sticks increases. expensive. Therefore, copper-based nanomaterials hold great promise as cheap and scalable transparent electrodes. As a (1) l πN c = 4.236. result, research has been focused toward designing and conFrom this theory, longer and thinner Ag NWs with smooth structing copper-based one-dimensional nanostructures. Zeng surfaces are preferred. Various deposition methods have been et al. demonstrated that copper nanowires (Cu NWs) can be explored for fabricating Ag NW electrodes from solution synthesized by a simple room-temperature solution process.20 The synthesized Cu NWs have a single crystalline structure such as Mayer rod coating, filtration, and drop coating. Figure 2c

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field to draw fine fibers from a liquid source (Figure 3c). Electrospinning has been explored as a fast and efficient process to fabricate continuous 1D nanomaterials.23 The fabrication process for electrospinning Cu nanofiber transparent electrode is described on the right side of Figure 3c. In step 1, precursor nanofibers with copper acetate dissolved in poly(vinyl acetate) (PVA) are electrospun onto a glass substrate, the fibers have diameters of around 200 nm. In step 2, polymer nanofibers with copper precursors are heated at 500°C in air for 2 h to remove all the polymer components, and the nanofibers are transformed to dark brown CuO nanofibers. In step 3, CuO nanofibers are reduced into red Cu nanofibers after annealing in an H2 atmosphere at 300°C for 1 h. The length of electrospun Cu NFs can be much larger than solution-processed Cu NWs. Figure 3d shows a single electrospun copper NF, with a diameter of 80 nm and a length >100 um. Typically, values for Rs and T are 200 Ω/sq at ∼96%, 50 Ω/sq at ∼90%, and 12 Ω/sq at ∼80% for Cu NF transparent electrodes at room temperature. Electrospinning can also provide a facile process to align NFs to form regular arrays (Figure 3e). In such directional NF systems, the charge carriers Figure 2. (a) Ag nanowire (NW) ink in ethanol solvent with a concentration of 2.7 mg/mL. are transported primarily along the fibers with (b) The Ag NW length and diameter histogram. (c) Scanning electron microscopy images of little junction scattering. The nearly junction an Ag NW mesh on a plastic substrate. The red dashed line indicates the electron transport resistance-free network with oriented NFs can path. (d) Electrical measurement of individual Ag NWs and a junction. (e) Sheet resistance versus annealing time for an Ag NW mesh annealed at 200°C. (f) Optical transmittance of greatly enhance the surface conductance in the transparent Ag NW electrodes measured with a UV-vis spectrometer, without including orientation direction with a shorter conduction the substrates. Indium tin oxide on poly(ethylene terephthalate) is obtained from CPFilms. path compared with a random network. Due (g) Sheet resistance Rs, σdc, and σdc/σop, where σdc is the dc conductivity and σop is the optical conductivity, for different film thickness. to the metallic nature of these films, light scattering is significant. There are two types of with diameters of 90 ± 10 nm and a length ∼10 um. However, transmittance: specular transmittance (the normal and forward in Zeng’s work, only a limited amount of Cu NWs (1 g transparent Cu NF electrodes show higher diffusive transmitof Cu NWs was synthesized by solution processing. The copper tances than specular transmittances. The difference is 10% at NWs were then printed on glass substrates to form a transparent ∼80% specular transmittance and 4% at ∼90% specular transconducting layer. The Cu NW film transparent electrodes have mittance (Figure 3f). Such transparent electrodes with large a sheet resistance of ∼20 Ω/sq at 60% transmittance. The low light scattering (Figure 3g) can help light absorption in the transmittance of Cu NWs can be attributed to their low aspect active layer, and therefore make Cu NF transparent electrodes ratio. Generally, a Cu NW has a diameter of ∼100 nm and a suitable for solar cell systems. length